Please refer to FIG. 1, which is a circuit diagram showing a non-isolated dc/ac converter applicable to an input voltage of a wide range according to the prior art. The dc/ac converter 100 includes a first stage dc/dc converter 102 and a second stage dc/ac converter 104.
The dc/dc converter 102 includes a dc input source 106, an inductor 108, a transistor switch 110, a diode 112 and a capacitor 114. The dc/dc converter 102 modulates a dc voltage Vdc from the dc input source 106 through the transistor switch 110 using pulse-width modulation to raise a bus voltage Vbus toward the second stage dc/ac converter 104, so that the bus voltage Vbus reaches a high and regulated voltage level.
The second stage dc/ac converter 104 includes four transistor switches 116, 118, 120 and 122, an inductor 124 and a capacitor 126. The second stage dc/ac converter 104 is electrically connected to the first stage dc/dc converter 102 to receive the bus voltage Vbus and then convert the bus voltage Vbus into an output ac voltage Vo at the load end with the switching of the transistor switches 116˜122 and the filtering of the filter composed of the inductor 124 and the capacitor 126
In the application of the non-isolated dc/ac converter shown in FIG. 1, the first stage dc/dc converter 102 raises the inputted dc source to a maximum value and regulates the inputted dc source. Then the second stage dc/ac converter 104 converts the dc source from the first stage dc/dc converter 102 into an ac source to provide the load.
The non-isolated dc/ac converter applicable to an input voltage of a wide range according to the prior art has some drawbacks as follows:
1. No matter the ac voltage at the output ends is high or low, the first stage dc/dc converter 102 must raise the inputted dc source to the voltage value higher than the peak of the outputted voltage. Then the voltage value higher than the peak of the outputted voltage will be kept. The second stage dc/ac converter 104 must be operated under a high voltage and the power loss therefore becomes more serious.
2. The first stage dc/dc converter 102 must continue to keep the Vbus higher than the peak of the outputted voltage, so the switches in the first stage dc/dc converter 102 switch frequently. The power loss is therefore increased.
3. The storage capacitor with a high capacitance, such as the capacitor 114 shown in FIG. 1, is adopted, so the cost is increased.